A process for preparing a force-displacement diagram of the press dies of a rotary tabletting press (rotary press), by which the compression characteristics of the material being pressed on a rotary press can be determined under production conditions with a minimum measuring effort, has been known from DE 195 02 596 A1. The course of the pressing force of at least one press die is measured at angular increments and stored in a computer. One revolution of the rotor corresponds to. e.g., 3,600 angular pulses. A force is assigned to each of these pulses and correspondingly stored in the computer. Furthermore, the theoretical values for the displacement of the press die are stored in the computer. These are to be calculated via corresponding geometric equations. The diameter of the press roll, the shape of the die head, and the relative position of these parts in relation to one another, are decisive for this theoretical value. A correction table, in which essential factors influencing the actual displacement of the die, such as the spring-back of the tabletting press and the flattening of the die and press roll due to the Hertzian stress or pressure, are taken into account, is stored in the computer. Both the spring-back and the Hertzian stress depend on the pressing force applied to the press die. Thus, force-dependent correction factors, which are to be deducted from the theoretical values for the die displacement to determine the actual die displacement, are determined. Since the die head has a radius in the circumferential area only, but is flat in the middle, the value of the Hertzian stress depends on the relative position of the die head in relation to the press roll. The correction values therefore take into account the dependence of the Hertzian flattening on the angular position of the press die. The vertical movement of a die pair can be calculated for each angle of rotation by mathematical methods, and the overall theoretical displacement must be corrected by the overall spring-back of the tabletting press and by the Hertzian flattening between the die head and the press roll.
This prior-art process is based on the usual head shape of dies for rotary presses, namely, on the circular, flat and flattened middle plateau surface and the edge area having a radius in its cross section. These prior-art die head shapes are sufficient for weak pressing forces to handle the pressing forces needed during continuous operation at the contact area between the press roll and the die. The die head has the circular plateau surface, which is joined by the toroidal intake surface. At the highest point of the toroidal surface, a punctiform contact becomes established at the maximum pressing force between the cylindrical press roll and the toroidal surface of the die head, which is arched in two planes, and consequently a relatively high Hertzian stress develops, which is the principal parameter for the force transmissible between the two bodies of the pair, namely, the die head and the press roll. The pressing force of the rotary press is limited as a result.
Based on the pitting occurring at high pressing forces in the die head-press roll pair, a limit of the permissible pressing forces has already been reached and sometimes exceeded with the prior-art die head shapes. The prior-art die head shapes with the punctiform contact can no longer be improved substantially for increasing the permissible pressing forces in future developments of the industrial rotary presses. If a pressing force of 80 kN and a prior-art die head shape are assumed, a Hertzian stress exceeding 4,300 N/mm.sup.2, which corresponds to a maximum permissible Hertzian stress for the pair formed by the die head and the press roll, is obtained at the transition of the toroidal surface into the plateau surface in the contact zone of the die head and the press roll with manufactured dies.